The older shape of Christmastide is the twelve days, from the Nativity on 25 December to the eve of Epiphany on 6 January, a span the church treated as festal from late antiquity; the Council of Tours in 567 already speaks of the days between the two feasts as a single festal tide. The twelve days gathered a dense cluster of observances.
The feast of Stephen, the first martyr, follows Christmas immediately on 26 December; John the Evangelist and the Holy Innocents follow on the two days after; the octave day of Christmas, 1 January, closes the feast's first week; and the whole tide runs out at Twelfth Night, the eve of Epiphany, which European custom kept with misrule, cakes and the crowning of bean kings. Much of the folklore of Christmas, the twelve days of the carol included, is the folklore of this span rather than of the single day.
The modern Roman calendar draws the season's boundary differently, ending Christmastide at the feast of the Baptism of the Lord, the Sunday after Epiphany, so the season now varies slightly in length with the calendar of a given year. The atlas records the season in both shapes, the folk and older liturgical twelve days and the current Roman definition, because both are still in living use, in churches and in customs that outlast their calendars.